Packaging systems comprising a rigid heat stable container having a thin flexible thermoplastic film sealed onto it are commonly used for the packaging of so-called “ready-meals”, that is meals that require only heating to be ready for consumption. Heating can be carried out either in a microwave or in a conventional oven. Due to the temperatures involved in the heating step only few materials can be used. Crystalline polyethylene terephthalate (CPET) containers are especially suitable for this application, where it is required to reheat the contents to high temperatures, typically in the order of 175° C. to 200° C. Whereas amorphous polyethylene terephthalate, i.e. a material which has a low degree of crystallinity, typically about 5 to 10%, becomes dimensionally unstable at approximately 70° C., CPET, which has a degree of crystallinity above 20%, remains dimensionally stable up to about 200° C. and above.
Containers made of a foamed material have some advantages over non-foamed ones in particular when ovenable applications are contemplated. Firstly, the foamed material has better heat insulating properties which allow manual handling of the packaged food product after heating in the oven. Secondly, as the density of the foamed material is lower, the amount of polymer resin needed to produce an article with a specific thickness and/or mechanical resistance is reduced. Thus, foamed CPET molded articles offer significant advantages as containers for food products which require heating in microwave or conventional ovens, i.e. containers for so-called “dual ovenable” applications.
Thermoforming is a process which is used commercially in the production of polyester articles. It is a particularly valuable technique for producing thin walled articles, such as dual ovenable food containers. However, it is well known in the art that polyester containers obtained by thermoforming sheets of high crystallinity are not heat stable, rather they warp and distort when heated in an oven.
Known methods to produce heat-resistant molded polyester foam products involve thermoforming a foamed polyester sheet of low crystallinity to produce a molded foam product whose degree of crystallinity is increased during the thermoforming process by molding the sheet with a heated molding die held at high temperature, and by continuously heating the polyester sheet inside the mold to induce crystallisation. The molded article is generally heat treated in the mold in order to attain a crystallinity of at least about 20%.
For instance, EP-A-115,162 discloses a method wherein a sheet of foamed polyester having a crystallinity in the range of from 0% to 18% is heated to a temperature between the glass transition and the melting temperature of the polyester resin; contacted with a heated mold which is held at a temperature which is preferably higher than the temperature of the sheet; the sheet is maintained in the heated mold until the average crystallinity is at least 20% and it is then removed from the mold.
EP-A-390,723 discloses a similar method wherein a substantially amorphous foamed polyester sheet is preheated above its glass transition temperature until is softens; it is drawn into a heated mold; heat-set by keeping the molded sheet in contact with the heated mold for a sufficient time to crystallise the sheet to a level of crystallinity of at least 15%.
U.S. Pat. No. 5,618,486 discloses a method for thermoforming foamed polyester sheets into heat-resistant containers wherein the low crystallinity sheet is preheated so that its surface temperature is between 110-150° C.; the preheated sheet is molded in a mold held at a temperature between 150° and 200° C. and held in said mold until the average crystallinity is at least 20%; the molded sheet is then advanced to and held in a cooling mold until it reaches a temperature of about 70° C.
The known methods for thermoforming heat-resistant foamed polyester articles require the use of a heated mold which is held at a temperature generally higher than the temperature of the foamed polyester sheet when it enters the mold. During the molding process heat is transferred from the heated mold to the foamed polyester sheet to increase its degree of crystallinity. The heat transfer occurs either during the thermoforming step or in a subsequent “heat-setting” step which takes place in a second mold.
It has now been found that it is possible to thermoform a foamed polyester sheet of low crystallinity to produce a molded foam article with a high degree of crystallinity, i.e. above 20%, without the need to provide heat to the polyester foam sheet during the thermoforming step or in a subsequent “heat-setting” step taking place in a second mold. In particular it has now been found that it is possible to thermoform a foamed polyester sheet of low crystallinity into a molded foam article with a degree of crystallinity above 20% by heating the foamed polyester sheet to a suitable preforming temperature and forming said sheet in a non-heated mold. The mold is at a temperature which is lower than the preforming temperature of the polyester sheet.
Thus a first object of the present invention is a process for thermoforming a foamed polyester sheet of low crystallinity into a heat-resistant molded foam article comprising the steps of: heating the foamed polyester sheet to a preforming temperature by contact with at least one heating plate; forming the heated foamed polyester sheet onto a non-heated mold; and releasing the molded foamed polyester sheet from the mold.
A second object of the present invention is a heat-resistant molded foam article obtainable with the process according to the first object.
The process of the present invention has the advantage that shorter cycle times and/or less bulky thermoforming equipment are required for the manufacture of heat-resistant foamed polyester molded products. Thus, as the container thermoforming section can be more compact, it is more convenient to integrate the container thermoforming step with a product packaging step and to carry out the whole process in-line on a single machine.
Thus a third object of the present invention is a packaging process comprising the steps of heating a foamed polyester sheet of low crystallinity to a preforming temperature by contact with at least one heating plate; forming the heated foamed polyester sheet onto a non-heated mold; releasing the heat-resistant molded foamed polyester sheet from the mold; loading a product into said heat-resistant molded foamed polyester sheet; bringing a film of thermoplastic material over said product and said heat-resistant molded foamed polyester sheet and sealing or welding said film of thermoplastic material onto said heat-resistant molded foamed polyester sheet.
A fourth object of the present invention is a package comprising a heat-resistant polyester molded foam article, a product loaded therein and a film of thermoplastic material placed over the product and the molded foam article and sealed or welded onto the molded foam article.
The heat-resistant molded foam articles and the packages of the present invention are particularly suitable for the packaging of food products for dual ovenable applications.
The same reference numbers will be used through the following description for indicating the same or functionally equivalent parts.